please stop losing my custom agent and go back to default agent.

please stop losing my custom agent and go back to default agent. I want my custom agent to stick to the session until I change it once I set one

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| Field | Value |
| --- | --- |
| App version | 1.0.17 |
| OS | Windows 10.0.26200 |
| Theme | GitHub |
| Path | /chat |
| Tenure | Day 1 |

Start in the app's /chat experience and reproduce the behavior described in the issue using the reported Windows environment and app version 1.0.17. Trace what happens to the selected custom agent during a session and verify that it remains selected until the user changes it; confirm that the default agent is used when no custom agent has been chosen.

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